Three prominent Republican senators have written to Attorney General Alberto R. Gonzales to express concern that Boeing may be allowed to take a tax deduction for a $615 million settlement reached last week with the government. The three senators — Charles E. Grassley of Iowa, chairman of the Senate Finance Committee; John McCain of Arizona, a longtime critic of Boeing; and John W. Warner, chairman of the Armed Services Committee — said in the letter that allowing Boeing to deduct payments to settle government ethics charges "would be unacceptable."